Fuel cell assemblies convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to separate electrodes that facilitate catalytic reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is typically a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in a fuel cell power system. A typical PEM for automotive application is 15-25 microns thick.
In a typical fuel cell assembly (or stack) within a fuel cell power system, individual fuel cell plates include channels through which various reactants and cooling fluids flow. Fuel cell plates are typically designed with straight or serpentine flow channels. Such flow channels are desirable as they effectively distribute reactants over an active area of an operating fuel cell, thereby maximizing performance and stability. In subzero temperatures, water vapor in the fuel cell assembly may condense. Further, the condensate may form ice in the fuel cell assembly. The presence of condensate and ice may affect the performance of the fuel cell assembly.
During typical operating conditions, condensate may also accumulate at the edges of the fuel cell plates adjacent outlet manifolds of the fuel cell assembly, thereby restricting fluid flow from the flow channels to the outlet manifolds. During a starting operation of the fuel cell assembly in subzero temperatures, the condensed water in the flow channels of the fuel cell plates and at the edges of the outlet manifolds is in the form of ice which may restrict reactant flow. Similarly, reactant flow maldistribution due to liquid water stagnation during normal operation can result.
Typically, to mitigate the formation of condensation at the outlet manifolds of the fuel cell assembly, the operating temperature of the fuel cell assembly is increased. However, increasing the operating temperature may have a negative impact on ohmic resistance due to increased membrane proton resistance as a result of decreased membrane humidification. Also, decreasing the inlet relative humidity of anode and cathode gas streams will achieve the same effect as increasing the operating temperature and result in a negative impact on ohmic resistance due to increased membrane proton resistance.
During operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the fuel cell assembly and mitigates water condensation and ice formation in the assembly. However, end plates of the fuel cell assembly tend to have a temperature lower than the temperature of intermediate plates of the fuel cell assembly. The end plates have a lower temperature due to thermal losses to the environment and thermal losses to terminal plates of the fuel cell assembly adjacent thereto. A difference in the temperature of the fuel cell plates throughout the fuel cell assembly may result in inefficient operation, maldistribution of reactants, condensation of water which may lead to ice formation, and a decreased useful life of the fuel cell assembly.
Typically, to ensure a substantially uniform temperature distribution between the plates in the fuel cell assembly, a heating mechanism is disposed adjacent the end plates to directly transfer thermal energy thereto. A heating mechanism may also be disposed adjacent the terminal plates to transfer thermal energy thereto. Thermal energy is then transferred from the terminal plates to the end plates. Alternatively, a resistive heating mechanism adapted to heat the end plates may be connected in parallel to the fuel cell assembly. If a heating mechanism fails and is in a powered state, the end fuel cells may dry out, thereby leading to an electrical short in the fuel cell assembly. Other methods for heating the end plates include catalytic heating, and providing a bypass plate disposed between the end plates and the terminal plates.
It would be desirable to develop a fuel cell assembly having a thermally insulating, electrically conducting layer disposed between a terminal plate and an end plate thereof to mitigate thermal losses from the end plate, and fluid condensation and ice formation on the end plate, and having end fuel cells with membranes and/or cathodes having a thickness greater than an average thickness of the membranes and cathodes used in the fuel cells in the remainder of the fuel cell assembly to further mitigate thermal losses from the end plate, and fluid condensation and ice formation in the end fuel cells.